1. NSC 243
Serotonin

2. B6 and B7 comprise dorsal raphe nuclei
B8 and B5 are the median raphe nuclei
B1-4 project to brainstem and cerebellum and spinal cord
95% of body’s 5-HT is found in the periphery in the enteric NS, about 100 million neurons. 5-HT is a major regulator of GI function—causes constriction of gastric smooth muscle

3. 5-HT staining of dorsal and median raphe nuclei
Rat midbrain

4. Very dense neo-cortical innervation; > NE
Proposed “harmonizer” activity, generally inhibitory, simultaneously affect activity throughout the brain; however, the m and d systems are distinct in several ways: electrophysiology, morhphology, and innervation. Generally increasing or decreasing 5-HT all over may not accomplish the goal. May need to be more regionally selective.
No system seems to absolutely require 5-HT
Raphe receive input from DA neurons in SN and VTA, and NE from LC; also reciprocal dorsal-median connections and afferents from amygdala, cortex, and hypothalamus
M system: coarse, large spherical varicosities
D system: fine, small varicosities, more sensitive to neurotoxic amphetamine derivatives like ecstasy

5. Very regular firing rate, regulated by Ca dependent K current, somatodendritic autoreceptors (5-HT1A), and heterosynaptic input.
Largely independent of stressors or environment

6. Functional Roles of 5-HT
General tone of NS
Repetitive activities like grooming activate 5-HT neurons
5-HT stimulates CRH (corticotropin-releasing hormone) release from hypothalamus
5-HT has negative effects on the ability of light to reset the circadian clock
5-HT activation decreases food consumption and drugs that inhibit 5-HT stimulate food intake
Effect on satiety

7. Serotonin synthesis
Quite analogous to DA production

8. Synthesis takes place in the terminal

9. Stimulation induced regulation of 5-HT production
This increase is Ca dependent, probably linked to phosphorylation, like TH. Kinetic changes not known (Km or Vmax)

10. Increasing tryptophan increases 5-HT release
Tryptophan availability is the limiting factor

11. Effect of egg protein diet on tryptophan:LNAA ratio
After different concentrations of hydrolyzed egg protein diet. This effect is mimicked by high carb diets, but not all protein diets, which are expected to increase all LNAA (large neutral amino acids)
British Journal of Nutrition (2011), 105, 611–617

12. Inactivation of 5-HT signal
SERT

13. Serotonin inactivation is a target of many pharmaceuticals
Inhibitors of 5-HT uptake and degradation are among the most common pharmaceuticals
Solid lines are agonists, dashed are inhibitors or antagonists * *

14. Serotonin transporters
Fenfluramine disrupts the proton gradient, blocks uptake of serotonin into vesicles, and also inhibits SERT uptake of 5-HT. Also makes SERT run in reverse—build up of 5-HT inside the cell favors export of 5-HT via SERT into the synapse.
Chronic inhibition of SERT (cocaine) causes its downregulation.
SERT activity regulated by activity in both directions

15. Inhibition of SERT
Tricyclic antidepressants like imipramine, SSRI’s like prozac
Inhibit SERT, increase 5-HT levels
Effects take weeks to develop
Compensatory changes?

16. Transient regulation of monoamine transporters

17. Regulation of 5-HT release
Tryptophan availability
Firing rate of neuron (pretty constant)
Autoreceptors
Somatodendritic
Terminal

18. Effect of 5-HT autoreceptors* *

19. Regulation of 5-HT release
Autoreceptors
Somatodendritic
“activity modulating”
Terminal
“release modulating”
Drug effects: tryptophan, SSRI, MAO inhibitors
Meant to increase 5-HT; actually can inhibit raphe firing via autoreceptors!

20. Regulation of 5-HT release
Effects of Drugs: Amph, Ecstasy (MDMA), and fenfluramine
Reverse SERT, block vesicular transport
Destroy dorsal raphe terminals (not median)
Tax energy: disrupt ion gradients
Non-exocytotic, non-Ca2+ dependent release

21. Toxic effects of MDMA

22. Activity of pre-synaptic autoreceptor 5HT1A on firing
In the absence of NK-1 (the substance P receptor), ipsapirone (the 5-HT1A agonist) is unable to inhibit 5-HT firing (IC50 is one order of magnitude higher). NK1 antagonists are useful antidepressants;  the constitutive lack of NK1 receptors appears to be associated with a downregulation/functional desensitization of 5-HT1Aautoreceptors resembling that induced by chronic treatment with SSRI antidepressants.
(5HT1A agonist)
Froger et al 2001

23. Release of CA
Ca2+ dependent exocytosis
Release at varicosities

24. Volume transmission

26. Sites of preferential volume release of 5-HT
Dorsal Raphe
SN? Protected by glial sheaths surrounding synapses, so not in practice
SERT located far from synapses in these regions, allows for distance transmission
Explains why receptors are often non-junctional